1. Technical Field
This application is directed to a system for detecting noise, particularly uncorrelated noise, via a microphone array and to a system for reducing noise, particularly uncorrelated noise, received by a microphone array connected to a beamformer.
2. Related Art
In different areas, handsfree systems are used for many different applications. In particular, handsfree telephone systems and speech control systems are getting more and more common for vehicles. This may be due to a perceived increase in comfort and safety that is obtained when using handsfree systems. Particularly in the case of vehicular applications, one or several microphones can be mounted in the vehicular cabin. Alternatively, a user can be provided with a corresponding headset.
However, in handsfree systems, the signal to noise ratio (SNR) usually is deteriorated (i.e., reduced) in comparison to a handset system. This is mainly due to the distance between the microphone and the speaker, and the resulting low signal level at the microphone. Furthermore, a high ambient noise level is often present, requiring utilization of methods for noise reduction. These methods are based on a processing of the signals received by the microphones. One channel and multi-channel noise reduction methods may be distinguished depending on the number of microphones.
Beamforming methods are used for background noise reduction, particularly in the field of vehicular handsfree systems, but also in other applications. A beamformer processes signals emanating from a microphone array to obtain a combined signal in such a way that signal components coming from a direction different from a predetermined wanted signal direction are suppressed. Microphone arrays, unlike conventional directional microphones, are electronically steerable which gives them the ability to acquire a high-quality signal or signals from a desired direction or directions while attenuating off-axis noise or interference.
Beamforming, therefore, may provide a specific directivity pattern for a microphone array. In the case of, for example, delay-and-sum beamforming (DSBF), beamforming encompasses delay compensation and summing of the signals. Due to spatial filtering obtained by a microphone array with a corresponding beamformer, it is often possible to improve the SNR. However, achieving a significant improvement in SNR with simple DSBF requires an impractical number of microphones, even under idealized noise conditions. Another beamformer type is the adaptive beamformer. Traditional adaptive beamformers optimize a set of channel filters under some set of constraints. These techniques do well in narrowband, far-field applications and where the signal of interest generally has stationary statistics. However, traditional adaptive beamformers are not necessarily as well suited for use in speech applications where, for example, the signal of interest has a wide bandwith, the signal of interest is non-stationary, interfering signals also have a wide bandwith, interfering signals may be spatially distributed, or interfering signals are non-stationary. A particular adaptive array is the generalized sidelobe canceler (GSC). The GSC uses an adaptive array structure to measure a noise-only signal which is then canceled from the beamformer output. However, obtaining a noise measurement that is free from signal leakage, especially in reverberant environments, is generally where the difficulty lies in implementing a robust and effective GSC. An example of a beamformer with a GSC structure is described in L. J. Griffiths & C. W. Jim, An Alternative Approach to Linearly Constrained Adaptive Beamforming, in IEEE Transactions on Antennas and Propagation, 1982 pp. 27-34.
In addition to ambient noise, the signal quality of a wanted signal can also be reduced due to wind perturbation. These perturbations arise if wind hits the microphone enclosure. The wind pressure and air turbulences may deviate the membrane of the microphone considerably, resulting in strong pulse-like disturbances, which may be known as wind noise or Popp noise. In vehicles, this problem may arise if the fan is switched on or in the case of the open top of a cabriolet.
For reduction of these perturbations, corresponding microphones are usually provided with a wind shield (also known as a “Popp shield”). The wind shield reduces the wind speed and, thus, also the wind noise without considerably affecting the signal quality. However, the effectiveness of such a wind shield depends on its size and, hence, increases the overall size of the microphone. A large microphone is often undesired because of design reasons and lack of space. Because of these and other reasons, many microphones are not equipped with an adequate wind shield, thereby resulting in poor speech quality for a handsfree device and low speech recognition rate of a speech control system.
Therefore, a need exists for a system for detecting and reducing noise and in particular uncorrelated noise such as wind noise at microphones.